Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy, which typically uses a therapeutic radiation source, such as a linear accelerator (LINAC), to generate radiation beams, such as x-rays. In one type of external beam radiation therapy, a therapeutic radiation source directs a sequence of x-ray beams at a tumor site from multiple co-planar angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the therapeutic radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to and from the tumor. As a result, the cumulative radiation dose at the tumor is high and that to healthy tissue is relatively low.
The term “radiosurgery” refers to a procedure in which radiation is applied to a target region at doses sufficient to necrotize a pathology in fewer treatment sessions or fractions than with delivery of lower doses per fraction in a larger number of fractions. Radiosurgery is typically characterized, as distinguished from radiotherapy, by relatively high radiation doses per fraction (e.g., 500-2000 centiGray), extended treatment times per fraction (e.g., 30-60 minutes per treatment), and hypo-fractionation (e.g., one to five fractions or treatment days). Radiotherapy is typically characterized by a low dose per fraction (e.g., 100-200 centiGray), shorter fraction times (e.g., 10 to 30 minutes per treatment) and hyper-fractionation (e.g., 30 to 45 fractions). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted.
Image-guided radiation therapy (IGRT) systems include gantry-based systems and robotic arm-based systems. In gantry-based systems, a gantry rotates the therapeutic radiation source around an axis passing through the isocenter. Gantry-based systems include C-arm gantries, in which the therapeutic radiation source is mounted, in a cantilever-like manner, over and rotates about the axis passing through the isocenter. Gantry-based systems further include ring gantries having generally toroidal shapes in which the patient's body extends through a bore of the ring/toroid, and the therapeutic radiation source is mounted on the perimeter of the ring and rotates about the axis passing through the isocenter. Traditional gantry systems (ring or C-arm) deliver therapeutic radiation in single plane (i.e., co-planar) defined by the rotational trajectory of the radiation source. Examples of C-arm systems are manufactured by Siemens of Germany and Varian Medical Systems of California. In robotic arm-based systems, the therapeutic radiation source is mounted on an articulated robotic arm that extends over and around the patient, the robotic arm being configured to provide at least five degrees of freedom. Robotic arm-based systems provide the capability to deliver therapeutic radiation from multiple out-of-plane directions, i.e., are capable of non-coplanar delivery. Accuray Incorporated of California manufactures a system with a radiation source mounted on a robotic arm for non-coplanar delivery of radiation beams.
Associated with each radiation therapy system is an imaging system to provide in-treatment images that are used to set up and, in some examples, guide the radiation delivery procedure and track in-treatment target motion. Portal imaging systems place a detector opposite the therapeutic source to image the patient for setup and in-treatment images, while other approaches utilize distinct, independent image radiation source(s) and detector(s) for the patient set-up and in-treatment images. Target or target volume tracking during treatment is accomplished by comparing in-treatment images to pre-treatment image information. Pre-treatment image information may comprise, for example, computed tomography (CT) data, cone-beam CT data, magnetic resonance imaging (MRI) data, positron emission tomography (PET) data or 3D rotational angiography (3DRA) data, and any information obtained from these imaging modalities (for example and without limitation digitally reconstructed radiographs or DRRs).
In one common scenario, the therapeutic source is a linear accelerator (LINAC) producing therapeutic radiation (which can be termed an “MV source”) and the imaging system comprises one or more independent x-ray imaging sources producing relatively low intensity lower energy imaging radiation (each of which can be termed a “kV source”). In-treatment images can comprise one or more (preferably two) two-dimensional images (typically x-ray) acquired at one or more different points of view (e.g., stereoscopic x-ray images), and are compared with two-dimensional DRRs derived from the three dimensional pre-treatment image information. A DRR is a synthetic x-ray image generated by casting hypothetical x-rays through the 3D imaging data, where the direction and orientation of the hypothetical x-rays simulate the geometry of the in-treatment x-ray imaging system. The resulting DRR then has approximately the same scale and point of view as the in-treatment x-ray imaging system, and can be compared with the in-treatment x-ray images to determine the position and orientation of the target, which is then used to guide delivery of radiation to the target.
There are two general goals in radiation therapy: (i) to deliver a highly conformal dose distribution to the target volume; and (ii) to deliver treatment beams with high accuracy throughout every treatment fraction. A third goal is to accomplish the two general goals in as little time per fraction as possible. Delivering an increased conformal dose distribution requires, for example, the ability to deliver non-coplanar beams. Delivering treatment beams accurately requires the ability to track the location of the target volume intrafraction. The ability to increase delivery speed requires the ability to accurately, precisely, and quickly move the radiation source without hitting other objects in the room or the patient, or violating regulatory agency speed limitations.
One or more issues arise with respect to known radiation therapy systems that are at least partially addressed by one or more of the preferred embodiments described further hereinbelow. Generally speaking, these issues are brought about by a tension in known radiation therapy systems between mechanical stability and system versatility, a tension that becomes more pronounced as the desired use of radiation therapy expands from head-only applications to applications throughout the body, such as (without limitation) the lungs, liver, and prostate. Robot arm-based systems tend to allow for larger ranges of radiation beam angles for different body parts than ring or C-arm gantry-based systems, especially when it is desired to keep the patient couch motionless during the radiation therapy session. Accordingly, robot arm-based systems generally tend to allow for more versatility in the kinds of therapy plans that may be available to the patient in comparison to C-arm and ring gantry-based systems. Further in view of the very heavy nature of most therapeutic radiations sources, which can weigh hundreds of kilograms, systems based on mounting of the therapeutic radiation source on a C-arm gantry suffer from undesired in-treatment deformation of the mount structures, which deformation is difficult to model or predict and leads to beam delivery errors and/or increased therapy planning margins due to the inability to precisely and accurately identify where the beam is pointed in three-dimensional space.
Ring gantry-based systems, on the other hand, tend to exhibit relatively high mechanical stability, i.e., less of the deformation problems exhibited by C-arm gantry-based systems, and thus can reproducibly and accurately position the radiation source, including doing so at relatively high mechanical drive speeds. However, as discussed above, gantry-based systems (like C-arm systems) tend to provide a lesser range of achievable angles for the introduction of therapeutic radiation into different body parts and, therefore, provide a narrower array of radiation treatment options as compared to robot arm-based systems.
X-ray tomosynthesis refers to the process of acquiring a number of two-dimensional x-ray projection images of a target volume using x-rays that are incident upon the target volume at a respective number of different angles, followed by the mathematical processing of the two-dimensional x-ray projection images to yield a set of one or more tomosynthesis reconstructed images representative of one or more respective slices of the target volume, wherein the number of x-ray projection images is less than that in a set that would be required for CT image reconstruction, and/or the number or range of incident radiation angles is less than would be used in a CT imaging procedure. Commonly, a plurality of tomosynthesis reconstructed images are generated, each being representative of a different slice of the target volume, and therefore a set of tomosynthesis reconstructed images is sometimes referred to as a tomosynthesis volume. As used herein, the term tomosynthesis projection image refers to one of the two-dimensional x-ray projection images acquired during the tomosynthesis imaging process.
For purposes of the above terminology, for some preferred embodiments, a set of images that is required for CT image reconstruction is considered to include images (e.g., 300 or more) generated over a range of incident angles that is 180 degrees plus the fan beam angle. For some preferred embodiments, the x-ray projection images for constructing a tomosynthesis image are taken over an angular range between 1 degree and an angular range value that is less than that needed for a complete projection set for CT imaging (e.g., 180 degrees plus the fan angle), wherein the number of projection images generated in this range is a value that is between 2 and 1000. In other preferred embodiments, the x-ray projection images for constructing a tomosynthesis image are taken over an angular range of between 5 degrees and 45 degrees, wherein the number of projection images generated in this range is between 5 and 100.
X-ray tomosynthesis has been proposed as an in-treatment kV imaging modality for use in conjunction with radiation treatment systems. In U.S. Pat. No. 7,532,705B2 it is proposed to process the three-dimensional pre-treatment image information (e.g., a planning CT image volume) to generate digital tomosynthesis (DTS) reference image data of a target located within or on a patient, such as by simulating x-ray cone-beam projections through the planning CT image volume. Subsequently, with the patient on the treatment bed, DTS verification images are generated by acquiring a number of x-ray cone beam images at different angles. Target localization is then performed by comparing landmarks, such as bony structures, soft-tissue anatomy, implanted targets, and skin contours in the DTS reference image data and DTS verification image data. In U.S. Pat. No. 7,711,087B2 it is proposed to acquire tomosynthesis image data during a treatment session. For purposes of movement tracking during the treatment session, tomosynthesis reconstructed slices are processed directly in conjunction with reference CT data in a process that searches for a tomosynthesis reconstructed image that best matches a selected reference CT slice. The identity of the particular tomosynthesis reconstructed image that yields a maximum degree of match, together with the amount of spatial offset required for that tomosynthesis reconstructed image to achieve the peak match, is used to localize the target in three-dimensional space. The commonly assigned U.S. Pat. No. 6,778,850, which is incorporated by reference herein, also discloses the use of x-ray tomosynthesis images (more particularly, the use of relatively low clarity intra-treatment 3D images of the target region synthesized from a plurality of 2D diagnostic images acquired at different angles) of as an in-treatment kV imaging modality.
Cone beam CT (CBCT) has also been proposed as an in-treatment imaging modality for use in conjunction with radiation treatment systems, in some cases as a kV imaging modality and in other cases as an MV (portal) imaging modality. Whereas conventional CT imaging reconstructs 2D slices from 1D projections through a target volume, the 2D slices then being stacked to form a 3D volumetric image, CBCT imaging directly constructs a 3D volumetric image from 2D projections of the target volume. As known in the art, CBCT offers the ability to form a 3D image volume from a single gantry rotation (more specifically, a rotation of at least 180 degrees plus a fan beam angle) about the target volume, whereas conventional CT requires one rotation per slice (for single-row detectors) or 1/M rotations per slice (for newer quasi-linear multi-row detectors having M rows). CBCT also provides for a more isotropic spatial resolution, whereas conventional CT limits the spatial resolution in the longitudinal direction to the slice thickness. However, because conventional CT systems usually offer a substantially higher degree of collimation near their linear or quasi-linear row detectors than can usually be afforded by CBCT systems near their two-dimensional detectors, scattering noise and artifacts are more of a problem for CBCT systems than for conventional CT systems.
In U.S. Pat. No. 7,471,765B2 it is proposed to use a CBCT imaging system including a kV x-ray tube and a flat-panel imaging detector mounted on a LINAC gantry such that the kV radiation is approximately orthogonal to the MV treatment radiation from the LINAC. Prior to treatment, a CBCT planning image is acquired for treatment planning. Subsequently, before each treatment fraction, a CBCT image is acquired and compared to the CBCT pre-treatment planning image, and the results of the comparison are used to modify the treatment plan for that treatment fraction to compensate for interfraction setup errors and/or interfraction organ motion. Due to limitations in permissible gantry rotation speeds (e.g., one rotation per minute) which cause the CBCT acquisition time to be slow compared to breathing (or other physiological cycles) of the patient, a gating scheme synchronized to patient breathing (or other physiological cycles) is used during CBCT acquisition to reduce the deleterious effects of organ motion in the reconstructed images. Also due to the relatively slow CBCT acquisition time, the CBCT volume data is generally useful only for patient set-up before each treatment fraction, and not for intra-fraction motion correction.
X-ray source arrays such as field emission “cold cathode” x-ray source arrays represent a promising advance in medical imaging and offer potential advantages over conventional x-ray tube sources in several respects. A conventional x-ray tube usually comprises a tungsten, tantalum or rhenium cathode that is heated to approximately 2000° C. to cause electrons to be emitted thermionically, the free electrons then being accelerated toward an anode by a high electrical potential such as 120 kV. X-ray radiation usable for imaging is created when the thermionically generated electrons strike an anode, usually made of tungsten, molybdenum, or copper, at a focal spot of the x-ray tube, the collision causing the emission of x-ray photons. While historically being the only practical and cost-effective way to provide imaging x-ray radiation in medical imaging environments, conventional x-ray tube sources can bring about many design compromises in view of their relatively large size and weight, high operating temperatures, high power consumption, relatively modest temporal resolution (e.g., on/off switching times), and their minimal amenability to miniaturization or formation into closely spaced arrays.
As an alternative to conventional x-ray tube technology in which free electrons are generated by thermionic emission, alternative technologies have been introduced in which the free electrons are generated by field emission. In a field emission source, free electrons are emitted upon the application of a voltage to a material having a high emission density, such as certain carbon nanotube (CNT) materials. Because field emission of electrons is produced by a high electric field, no heating is necessary. Field emission sources are thus often referred to as cold cathode sources. Advantageously, the electron beams emitted by such materials may have low divergence and thus provide ease of focusing onto a focal spot. Moreover, the virtually instantaneous response of the source offers time gating capabilities that may even be on the order of nanoseconds. Because they can be made exceedingly small, field emission x-ray sources are highly amenable to formation into arrays. According to U.S. Pat. No. 7,505,562B2, which is incorporated by reference herein, devices having 1000 pixels per meter (i.e., 1000 individual x-ray sources per meter) with pulse repetition rates on the order of 10 MHz can be envisioned using technology within the current state of the art.
As used herein, the term x-ray source array refers to a source of x-rays comprising a plurality of spatially distinct, electronically activatible x-ray emitters or emission spots (focal spots) that are addressable on at least one of an individual and groupwise basis. Although most x-ray source arrays suitable for use with one or more of the preferred embodiments will commonly be of the field emission “cold cathode” type, the scope of the present teachings is not so limited. By way of example, other types of x-ray source arrays that may be suitable for use with one or more of the preferred embodiments include scanning-beam array X-ray sources in which an electron beam digitally scans across a tungsten transmission target thirty times per second, sequentially producing ten thousand individually collimated X-ray beams, as reported by Triple Ring Technologies, Inc., of Newark, Calif.
X-ray source arrays have been proposed for use in kV imaging systems associated with radiation treatment systems, such as in US20090296886A1. However, it is believed that substantial advances in the configuration, operation, and/or manner of integration of x-ray source arrays into IGRT systems, such as those provided by one or more of the preferred embodiments herein, are needed in order to achieve clinical practicality, effectiveness, and market acceptance. It is to be appreciated the although particularly advantageous in the context of IGRT systems, one or more of the preferred embodiments is also applicable to a wide variety of other medical imaging applications outside the realm of image-guided radiation treatment.
More generally, one or more issues arises with respect to known medical imaging and/or radiation treatment systems that is at least partially addressed by one or more of the preferred embodiments described further hereinbelow. Other issues arise as would be apparent to a person skilled in the art in view of the present teachings.